1. Field of the Invention
The invention relates generally to the field of sample preparation. More particularly, the invention relates to methods, systems and devices for concentrating a substance within a fluid sample.
2. Background of the Invention
The difficulties of detecting and quantifying particles in air and liquids are well known. Existing systems all begin to fail as concentration falls away until eventually, with diminished concentrations of analyte, there is an inability to detect at all. This poses a significant problem for national security where, for example, the postal anthrax attacks of 2001 and the subsequent war on terrorism have revealed shortcomings in the sampling and detection of biothreats. The medical arts are similarly affected by the existing limits on detection, as are the environmental sciences.
In the fields of biodefense and aerosol research it is common to collect aerosols into a liquid sample using a wet cyclone or similar device. The aerosol is collected into an aqueous sample so that subsequent analysis of biological particles can be performed using standard techniques that primarily require that the sample be contained in liquid. These “wet” collectors have many failings, including: difficulty in maintaining a set fluid volume, difficulties with buildup of particle matter in the device, and requirements for storage of the fluid in varying environmental conditions.
Dry filters have long been used for collection of aerosols, as well as for collection of particles from liquids. However, dry filters fail primarily for the use of identifying biological particles because detectors generally require a liquid sample and it is extremely difficult to remove the particles into a liquid. Methods for removing particles from flat filters are common but are tedious, inefficient, and require large liquid volumes.
Concentration of particles from a liquid is traditionally performed using centrifugation. Centrifugal force is used for the separation of mixtures according to differences in the density of the individual components present in the mixture. This force separates a mixture forming a pellet of relatively dense material at the bottom of the tube. The remaining solution, referred to as the supernate or supernatant liquid, may then be carefully decanted from the tube without disturbing the pellet, or withdrawn using a Pasteur pipette. The rate of centrifugation is specified by the acceleration applied to the sample, and is typically measured in revolutions per minute (RPM) or g-forces. The particle settling velocity in centrifugation is a function of the particle's size and shape, centrifugal acceleration, the volume fraction of solids present, the density difference between the particle and the liquid, and viscosity of the liquid.
Problems with the centrifugation technique limit its applicability. The settling velocity of particles in the micron size range is quite low. Consequently, centrifugal concentration of these particles takes several minutes to several hours. The actual time varies depending on the volume of the sample, the equipment used, and the skill of the operator.
Centrifugation techniques are tedious in that they are normally made up of multiple steps each requiring a high level of concentration from the operator. Most microbiology laboratories process large numbers of samples by centrifugation on a daily basis. The potential for human error is high due to the tedious nature and automation of these techniques is difficult and costly. Centrifugation also generally requires powered equipment. Thus, many situations, such as emergency response, prevent their use.
Other concentration techniques have been explored and primarily fall into three technology groups—microfluidic/electrophoretic based, filtration based, and capture based. However, each of these techniques has disadvantages that prevent their use in certain situations.
What is needed is a single device for concentrating a fluid sample. Such a device should be able to operate without electrical power to enable use in diverse situations.